Inbreeding Depression: why close crossing weakens plants
In short
Inbreeding depression is the loss of performance that appears when plants are crossed closely over several generations. Vigour, fertility and yield decline. Darwin already described the effect across more than fifty plant species.
The mechanism behind it
Every population carries a set of harmful, recessive traits. As long as a plant is mixed at the spot in question, the harmful trait stays covered by the healthy one and has no effect. Close crossing and selfing, though, raise the uniformity of the genome. More and more spots come into double copy, and the harmful traits appear in pairs and show their effect. That is the core of inbreeding depression: more homozygosity exposes existing harmful alleles.
The reverse: purging and heterosis
The effect has two counterparts. Because close crossing makes the harmful alleles visible, selection can remove them on purpose, a process called purging. And crossing two different inbred lines back together lets their healthy traits complement each other, so the offspring are often more vigorous than either parent. This gain is called heterosis, or hybrid vigour.
Link to cannabis breeding
In cannabis breeding, inbreeding depression is the flip side of selfing and line breeding. Stabilise a line through continued selfing and you risk weaker plants with each generation, unless you deliberately select against the harmful traits. That is why breeders switch on purpose between narrowing and outcrossing.